The present invention relates generally to high frequency filters and more particularly to an improved hybrid filter structure fabricated with plug-in resonant elements (elements of microstrip) and which filter may be tuned to a desired response after manufacture. This type of filter could be utilized in a high frequency (e.g. 400 Mhz) radio receiver to provide such functions as an injection filter or a preselector filter. The invention could also be used advantageously as a harmonic filter in a radio transmitter.
Filters are classified according to function (i.e., bandpass, lowpass, and high pass) and can be implemented in several common technologies. Typically filters are constructed of discrete components, or fabricated using printed circuit techniques such as stripline or microstrip. At the present time, microstrip filters are constructed using a planar approach, that is, the entire filter is fabricated on a single substrate, and since a microstrip filter is fabricated using printed circuit techniques, normally little can be done to tailor the filter's response after manufacture.
A major problem with microstrip filters in the past has been in coupling the individual resonators. Unlike many transmission lines, a microstrip resonator has separate and distinct effective dielectric constants depending on which coupled natural mode is excited. The electric flux distribution is distinctly different in each of these modes. An electromagnetic wave traveling through a filter with two coupled microstrip lines can exhibit two distinct propagation velocities depending on the excitation mode. The ultimate result of this phenemona is a high insertion loss for narrowband bandpass filters.
To overcome this problem, the present invention successfully separates the resonator and coupling sections. Since each resonator section comprises a ground plane, each resonator is self-shielding, and coupling between resonator sections can be minimized. By placing the coupling section on a separate substrate, in a different plane of orientation, the coupling effects are further reduced. The electromagnetic wave travels from resonator to resonator with a minimum of loss. Also, since the mounting substrate would typically be made of an inexpensive, relatively low dielectric material, a cost savings is realized by conserving the expensive high dielectric substrate material required for the resonator sections. Furthermore, since the air gap used in the coupling area is very small, the technique uses space efficiently.
The present invention overcomes the foregoing and other disadvantage and provides an improved microstrip filter which can be easily and inexpensively manufactured.